I bought a Sandisk 2GB Ultra II to use with my 650 but it isn't recognised by OSX 10.4 which I recently upgraded to. My 1GB SD card works fine although it is a different brand. Has anyone else had a similar problems?

There is no reason I'm aware of that it shouldn't be recognized. How are you connecting the card? Mounting with software, card reader? I know this is simple advice, but... you might try a restart to see if that solves the problem.